The World Health Organization estimates that 17 million people die every year of cardiovascular disease, and of heart attacks and stroke in particular. Strokes are caused by a blockage in the blood supply to the brain, either by a clot or a broken blood vessel. Most symptoms of stroke are obvious. But VOA's Melinda Smith has more on the "silent stroke" that often goes undiagnosed.

Most strokes are readily apparent by common symptoms such as weakness, numbness, confusion, loss of balance or a severe headache.

But recent studies show that many people experience strokes with no recognizable symptoms. They are called "silent strokes" and they frequently are not recognized either by the patient or the doctor.

They can eventually affect memory and set the stage for other illnesses years later. People who have had silent strokes typically score lower on tests of mental dexterity.

As many as 600,000 Americans are diagnosed with the more obvious forms of stroke. But a recent study of 16,000 people indicates that silent strokes are also common. At least one in three older people from age 70 on are believed to experience a silent stroke every year.

In another study, younger women who frequently suffer from migraine headaches, combined with the visual symptoms of flashing light or spots, are also at higher risk for stroke.

This type of headache is called 'migraine with aura.'

Women with aura migraines who smoke and use birth control pills are seven times more likely to have a stroke in the future. Researchers say they could reduce those odds by eliminating cigarettes and adopting a healthier lifestyle.